It is generally known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity and the radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wavelength and half wavelength antennas are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). Smaller packaging of state-of-the-art communications devices may not provide sufficient space for the conventional quarter and half wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing the other desirable antenna operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor. Increased gain thus requires a physically larger antenna, while communications device manufacturers and users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-frequency and/or wide bandwidth operation, allowing the communications device to access various wireless services operating within different frequency bands from a single antenna. Finally, gain is limited by the known relationship between the antenna frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter wavelength of the operating frequency.
The known Chu-Harrington relationship relates the size and bandwidth of an antenna. Generally, as the size decreases the antenna bandwidth also decreases. But to the contrary, as the capabilities of handset communications devices expand to provide for higher data rates and the reception of bandwidth intensive information (e.g., streaming video), the antenna bandwidth must be increased.
One basic antenna commonly used in many applications today is the halfwavelength dipole antenna. The radiation pattern is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
The quarter-wavelength monopole antenna placed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but with the ground plane the antenna performance resembles that of a half-wavelength dipole. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. However, conventional loop antennas are too large for handset applications and do not provide multi-band operation. As the loop length increases (i.e., approaching one free-space wavelength), the maximum of the field pattern shifts from the plane of the loop to the axis of the loop. Placing the loop antenna above a ground plane generally increases its directivity.
Given the advantageous performance of quarter and half wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency, and the antenna is operated over a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the operational frequency increases/decreases, the operational wavelength correspondingly decreases/increases. Since the antenna is designed to present a dimension that is a quarter or half wavelength at the operational frequency, when the operational frequency changes, the antenna is no longer operating at a resonant condition and antenna performance deteriorates.
As can be inferred from the above discussion of various antenna designs, each exhibits know advantages and disadvantages. The dipole antenna has a reasonably wide bandwidth and a relatively high antenna efficiency (or gain). The major drawback of the dipole, when considered for use in personal wireless communications devices, is its size. At an operational frequency of 900 MHz, the half-wave dipole comprises a linear radiator of about six inches in length. Clearly it is difficult to locate such an antenna in the small space envelope associated with today's handheld devices. By comparison, the patch antenna or the loop antenna over a ground plane present a lower profile resonant device than the dipole, but as discussed above, operate over a narrower bandwidth with a highly directional radiation pattern.
As discussed above, multi-band or wide bandwidth antenna operation is especially desirable for use with various personal or handheld communications devices. One approach to producing an antenna having multi-band capability is to design a single structure (such as a loop antenna) and rely upon the higher-order resonant frequencies of the loop structure to obtain a radiation capability in a higher frequency band. Another method employed to obtain multi-band performance uses two separate antennas, placed in proximity, with coupled inputs or feeds according to methods well known in the art. Thus each of the two separate antennas resonates at a predictable frequency to provide operation in at least two frequency bands. Notwithstanding these techniques, it remains difficult to realize an efficient antenna or antenna system that satisfies the multi-band/wide bandwidth operational features in a relatively small physical volume.
In an effort to overcome some of the disadvantages associated with the use of monopole, dipole, loop and patch antennas as discussed above, antenna designers have turned to the use of so-called slow wave structures where the antenna physical dimensions are not equal to its effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(∈r)sqrt(μr))=λf. Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light in a vacuum, the wavelength within the structure is lower than the free space wavelength. Thus, for example, a half wavelength slow wave structure is shorter than a half wavelength conventional structure where the wave propagates at the speed of light (c). The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength. Slow wave structures can be used as associated antenna elements (i.e., feeds) or as antenna radiating structures.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating the slow wave will be physically smaller than the structure propagating the wave at the speed of light.
Slow wave structures are discussed extensively by A. F. Harvey in his paper entitled Periodic and Guiding Structures at Microwave Frequencies, in the IRE Transactions on Microwave Theory and Techniques, January 1960, pp. 30-61 and in the book entitled Electromagnetic Slow Wave Systems by R. M. Bevensee published by John Wiley and Sons, copyright 1964. Both of these references are incorporated by reference herein.
A transmission line or conductive surface overlying a dielectric substrate exhibits slow-wave characteristics, such that the effective electrical length of the slow-wave structure is greater than its actual physical length, according to the equation,1e=(∈eff1/2)×1p,where 1e is the effective electrical length, 1p is the actual physical length, and ∈eff is the dielectric constant (∈r) of the dielectric material proximate the transmission line.
A prior art meanderline, which is one example of a slow wave structure, comprises a conductive pattern (i.e., a traveling wave structure) over a dielectric substrate, overlying a conductive ground plane. An antenna employing a meanderline structure, referred to as a meanderline-loaded antenna or a variable impedance transmission line (VITL) antenna, is disclosed in U.S. Pat. No. 5,790,080. The antenna consists of two vertical spaced apart conductors and a horizontal conductor disposed therebetween, with a gap separating each vertical conductor from the horizontal conductor.
The antenna further comprises one or more meanderline variable impedance transmission lines bridging the gap between the vertical conductor and each horizontal conductor. Each meanderline coupler is a slow wave transmission line structure carrying a traveling wave at a velocity lower than the free space velocity. Thus the effective electrical length of the slow wave structure is greater than its actual physical length. Consequently, smaller antenna elements can be employed to form an antenna having, for example, quarter-wavelength properties. As for all antenna structures, the antenna resonant condition is determined by the electrical length of the meanderlines plus the electrical length of the radiating elements.
The meanderline-loaded antenna allows the physical antenna dimensions to be reduced, while maintaining an effective electrical length that, in one embodiment, is a quarter wavelength multiple. The meanderline-loaded antennas operate near the known Chu-Harrington limits, that is,efficiency=FVQ,                where: Q=quality factor                    V=volume of the structure in cubic wavelengths            F=geometric form factor (F=64 for a cube or a sphere)Meanderline-loaded antennas achieve this efficiency limit of the Chu-Harrington relation while allowing the effective antenna length to be less than a quarter wavelength at the resonant frequency. Dimension reductions of 10 to 1 can be achieved when compared to a quarter wavelength monopole antenna, while achieving a comparable gain.                        